From the discovery of superconductivity in 1911 to the recent past, essentially all known superconducting materials were elemental metals (e.g., Hg, the first known superconductor) or metal alloys or intermetallic compounds (e.g., Nb.sub.3 Ge, probably the material with the highest transition temperature T.sub.c known prior to 1986).
Recently, superconductivity was discovered in a new class of materials, namely, metal oxides. See, for instance, J. G. Bednorz and K. A. Muller, Zeitschr. f. Physik B--Condensed Matter, Vol. 64, 189 (1986), which reports superconductivity in lanthanum barium copper oxide.
The above report stimulated worldwide research activity, which very quickly resulted in further significant progress. The progress has resulted, inter alia, to date in the discovery that compositions in the Y-Ba-Cu-O system can have superconductive transition temperatures T.sub.c above 77K., the boiling temperature of liquid N.sub.2 (see, for instance, M. K. Wu et al, Physical Review Letters, Vol. 58, Mar. 2, 1987, page 908; and P. H. Hor et al, ibid, page 911). Furthermore, it has resulted in the identification of the material phase that is responsible for the observed high temperature superconductivity, and in the discovery of composition and processing techniques that result in the formation of bulk samples of material that can be substantially single phase material and can have T.sub.c above 90K. (see, for instance, R. J. Cava et al, Physical Review Letters, Vol. 58(16), pp. 1676-1679), incorporated herein by reference., Vol. 58(16), pp. 1676-1679).
The excitement in the scientific and technical community that was created by the recent advances in superconductivity is at least in part due to the potentially immense technological impact of the availability of materials that are superconducting at temperatures that do not require refrigeration with expensive liquid He. Liquid nitrogen is generally considered to be one of the most advantageous cryogenic refrigerant, and attainment of superconductivity at liquid nitrogen temperature was a long-sought goal which until very recently appeared almost unreachable.
Although this goal has now been attained, there still exist barriers that have to be overcome before the new oxidic high T.sub.c superconductive materials can be utilized in many technological applications. In particular, techniques for forming high T.sub.c superconductive bodies of technologically significant shape have to be developed. Among the shapes of technological significance are normal metal-clad elongate bodies, e.g., wires and ribbons.
Prior art metallic superconductors have been prepared in wire and ribbon form, and have found use in, for instance, superconductive magnets. As is well known, superconductive wires and the like are almost invariably surrounded by a normal metal cladding, which provides, inter alia, an alternate current path in the event of a local loss of superconductivity,
For a general overview of some potential applications of superconductors see, for instance, B. B. Schwartz and S. Foner, editors, Superconductor Applications: SQUIDS and MACHINES, Plenum Press 1977; and S. Foner and B. B. Schwartz, editors, Superconductor Material Science, Metallurgy, Fabrications, and Applications, Plenum Press 1981. Among the applications are power transmission lines, rotating machinery, and superconductive magnets for e.g., fusion generators, MHD generators, particle accelerators, levitated vehicles, magnetic separation, and energy storage, as well as junction devices and detectors. It is expected that many of the above and other applications of superconductivity would materially benefit if high T.sub.c superconductive material could be used instead of the previously considered relatively low T.sub.c materials.
U.S. patent application Ser. No. 036,160, a continuation-in-part of U.S. patent application Ser. No. 034,117 ('117), now abandoned titled "Apparatus and Systems Comprising a Clad Superconductive Oxide Body, and Method for Producing Such Body," by S. Jin et al, and incorporated herein by reference, discloses a technique for producing normal-metal clad superconductive oxide wire and other elongate bodies. The technique comprises heat treating the clad elongate body. The oxides of concern herein lose oxygen at relatively high temperatures (such as are typically required for sintering), and can take up oxygen at intermediate temperatures, and the above patent application discloses various techniques for carrying out the heat treatment such that the oxygen content of the sintered oxide is in the range that is associated with superconductivity, and such that the oxide has the appropriate crystal structure. Among the suggested techniques is perforating, at appropriate intervals, the normal metal jacket that surrounds the oxide powder, such that the ambient oxygen can come into contact with the powder. See '117, page 8.
Even though '117 discloses several techniques that can produce elongate clad superconductive oxide bodies such as wires and ribbons, further simple techniques for producing such bodies that reliably permit relatively free access of oxygen to the superconductive oxide during heat treatment may still be of considerable technological and economic significance. This application discloses such a further technique.